Photodectors are sensors which can detect light or other electromagnetic energy. There are many different types of photodectors such as active pixel sensors. Active-pixel sensors (APSs) are image sensors fabricated using CMOS processes, and are thus also known as CMOS image sensors. Active pixel sensors are commonly used in cell phone cameras, web cameras, and some digital single-lens reflex cameras (DSLRs).
The photodectors can be fabricated with Ge material using CMOS processes. For example, fabrication of crystalline Ge photodetectors is by liquid phase epitaxy of amorphous Ge material, which requires encapsulation with a multilayer dielectric stack to form a crucible in which the Ge is melted during rapid thermal anneal. However, amorphous Ge has high density (small volume), whereas crystalline Ge has lower density (larger volume), resulting in Ge volume expansion inside the encapsulation stack during the anneal process. This expansion leads to the formation of cracks/breaks of the encapsulation layers which, in turn, causes Ge removal during subsequent wet etch processing.